Select the correct answer.

Read this excerpt from Robert Stawell Ball’s Great Astronomers:


Though every child may now know more of the actual truths of the celestial motions than ever Ptolemy knew, yet the fact that his work exercised such an astonishing effect on the human intellect for some sixty generations, shows that it must have been an extraordinary production. We must look into the career of this wonderful man to discover wherein lay the secret of that marvellous success which made him the unchallenged instructor of the human race for such a protracted period.


Based on the context of the excerpt, what does the word protracted most clearly mean?